Many have been left malnourished after being put on harmful restrictive diets by parents who have used companies who have wrongly diagnosed intolerances and allergies.

The NHS standards watchdog will say tomorrow that there is no evidence to support "alternative" tests to which many parents have turned in desperation, after their GPs failed to help.

The National Institute for Health and Clinical Excellence (NICE)'s first guidance on children's food allergies will say GPs need to act much more quickly to investigate common childhood complaints which could be triggered by diet.

The watchdog describes a 500 per cent increase in hospital admissions for food allergies in the UK since 1990. Its guidelines will cite research showing that up to 8 per cent of children under the age of three now suffer from some kind of food complaint.

While immediate allergies can be shown by blood or skin prick tests, establishing a delayed allergic reaction is much more difficult.

NICE will say "alternative" tests such as hair analysis, kinesiology, which measures muscle strength as a person lies next to a suspected allergen, and "vega" tests which measure electrical activity should not be used. The guidelines will also warn against the use of 'IgG' antibody tests promoted by commercial firms in clinics and via the internet.

Experts who drew up the guidelines said desperate parents turned to such methods because they appeared to offer certainty in diagnosing delayed reactions to food, when the NHS could only offer "a slow and laborious process" of eliminating foods one by one until the culprit was identified.

The draft guidelines, being published for consultation, will say GPs need to start such investigations sooner, and that the "alternative" tests being offered for delayed reactions should not be used.

Dr Adam Fox, consultant paediatric allergist from Guy's and St Thomas' Hospital, one of the experts who drew up the recommendations, said such tests often had dangerous consequences.

He said: "The NICE guideline will say that there is no evidence base for such approaches – and we think that is very helpful because at the moment parents are not only wasting time and money on them, but they also often end up putting their children on very extensive restriction diets following the diagnosis, which can leave them malnourished."

Dr Fox said the methods often inaccurately suggested intolerances to large groups of foods – such as wheat and dairy – leaving children missing out on vital nutrients.

The specialist said that while many parents feared their child had an allergy when they did not, many GPs were too quick to dismiss concerns, and were likely to miss many genuine cases.

"We are saying parents shouldn't be fobbed off, we do hear of mothers feeling their concerns are not taken seriously, and my view would be that you dismiss these concerns at your peril," he said.

"In cases of delayed allergy reactions we found there was very commonly a delay in diagnosis," added Dr Fox. "We are saying GPs should step in much sooner."

While most cases of children's allergies are dealt with by GPs, there are very few experts to deal with the most serious problems, with just 15 to 20 paediatric allergy specialists working in the UK.

The most common foods to which children are allergic include cows milk, fish and shellfish, hens eggs, peanuts, tree nuts, sesame seeds, soy, wheat and kiwi fruit.

NICE will say that where an immediate reaction to food is suspected – such as in the case of a child suffering hives immediately after eating peanuts – blood or skin prick tests should be used to confirm a diagnosis.

GPs will be told they should also investigate the possibility of a delayed reaction to food in every case where a child has an unexplained complaint of eczema, a rash, sneezing, vomiting, colic, gastro-oesphageal reflux or persistent diarrhoea.

Doctors will be told to take detailed family and personal histories to identify the likelihood of a food allergy, before putting children on a careful "exclusion diet" for between two to six weeks in an attempt to identify the suspected allergen.

Studies have found that 40 per cent of all children in the UK have some kind of allergy, when hay fever, asthma and eczema are counted with food reactions.

Scientists have been unable to establish the reasons for the increase, with some studies blaming changes in the environment, and others suggesting lack of exposure to germs and delays in weaning children on to solid foods may have played a part. Experts said studies on specific allergies, such as peanut reactions, had demonstrated that the increase could not be explained by improved awareness of the conditions.

The NICE guidelines will support the use of conventional antibody tests called immunoglobulin E (IgE) which are used by the NHS to detect immediate food reactions.

However, NICE will say such tests should only be interpreted by a doctor with a detailed clinical history of the patient – a warning against commercial firms which offer results by post.

It will say that a different type of test – immunoglobin G – offered by many private companies and used by many people trying to identify delayed reactions – does not detect allergies or intolerances.